As it happens, I think I disagree with you. I do value greatness. I value a culture that lauds greatness. The point of virtuosic musicianship isn't entertainment, or at least not a banal thoughtless kind (a symphony is not a substitute good for ragebait podcast clips with a subway surfers overlay), it's inspirational art. The examples I chose are particularly evocative, but there's no real difference between that and a parent who compels or allows their child to become ridiculously capable in some kind of mathematics or literature. Imagine if Terrence Tao's parents had insisted that he carry on with a typical pre-university series of broad survey courses for the sake of making him a generalist! Imagine all the less high-profile examples who were maybe even more important to pushing some practical effort forward.
Making it illegal is a nonstarter because I think it runs afoul of the categorical imperative in exactly the same way. I'm a strong believer in the idea that most progress (again, not intended to have a positive connotation) is made by a small group of people who were almost never generalists. Einstein was not a generalist. Kant, who I've been referencing throughout this conversation, was really not a generalist. The possibility of greatness is just as necessary as a certain number of pliable generalists.
What would the point of living in a world without greatness be? Since I meant that question rhetorically: is there a way to allow such greatness to be achieved without manipulating young people into obsession?